The overall goal of this research is to apply basic knowledge of binaural hearing to the solution of clinical problems. Specifically, the two goals of this work are: (1) to evaluate the binaural performance of individuals with various degrees and configurations of hearing loss fitted with different hearing aid configurations, and (2) to determine the extent to which psychoacoustic detection measurements (masking-level differences and contralateral masking) are capable of predicting speech intelligibility and localization ability with these hearing aid configurations. Hearing-aid evaluations will entail measuring word-intelligibility thresholds and source localization in quiet and in noise in both anechoic and reverberant environments. Both the acoustic sound fields and the hearing aids will be simulated by applying measured transfer functions (for the sound fields) and filters specified by most-comfortable-level adjustments (for the hearing aids). One configuration will be designed to optimize the use of binaural cues. Psychoacoustic detection tests will assess the ability to use binaural cues under the same hearing-aid configurations. The results of these detection tests will be used in a theoretical model to predict binaural and directional advantages in speech intelligibility. The outcome of this work will help to clarify understanding of the potential benefit of binaural hearing aids. Further, by establishing relations between aided binaural performance and simple detection measures, we will be laying the foundation for a clinical test that can be used to predict binaural performance with different hearing aids.